My '91 300TE 4matic decided the fan wasn't necessary anymore.
I'm trying to change its mind. Can anybody tell me an easy to get at access point to determine if the fan is getting voltage? I know the system well enough to know the problem could be in the fan motor, the speed controller mounted on the motor, or even the pushbutton unit in the dash. I removed the fan motor, replaced the brushes and relubed the bearings last year so I know how to get the motor out. I'm just trying to make sure I don't fix the part that isn't broke.

Fairly easy diagnostics. First be sure your fuse is good. Some cars have had their fuse moved out of the box to a single fuse holder similar to diesel glow fuses.

If the fuse is good (don't assume verify voltage at both ends), then you should look for the fan speed signal. This signal goes from the pushbutton controller to the blower regulator. It is easily available at a three wire connector just inboard of the brake booster on the firewall. It will be the yellow wire. The voltage on the wire will be from about 1v to 6-7v as you go from slow to high speed. In auto the speed signal will change as you change the temp. If this signal is good, expose the blower and reverify the power (red wire) side of the blower. The regulator controlls current in relationship to input voltage by regulating the ground side. I always like to jumper the ground side just to prove the motor will work, if I am condemning the regulator (which is below the motor).

OK, so I quickly ruled out the easy causes for no blower thanks to your help, Steve. The fuse is fine and the controller is putting out the right signal at the connection on the firewall.

Having been into the blower motor once before, it struck me as very funny how you were able to condense 2 hours of tedium with three words, "expose blower motor". The first time I took 5 hours or so in and out, including brush replacement and bearing lube. Hopefully I'll be able to get in and figure the problem out in an hour or two. I don't want to order the motor assembly until I know what isn't working.

Have you checked the ignition switch?
I had a similar problem on my 300E. The blower would blow in preignition, but cut out when I had started the engine. Turned out the blower motor and controller were fine the ignition switch was bad and wasn't feeding any power to the controller.
Just thinking this might also help. The ignition switch is a faily cheap and easy to replace part.
Good luck.
F.J

I'll check tomorrow, FJ, but the whole ignition switch, steering column lock, and tumbler assembly were replaced at 160 k miles. That's only 50 k miles ago, so I'd be surprised if it failed that quickly. Thanks for the suggestion!

If you have voltage and the correct signal at the connector next to the brake booster, but your fan is still not working, then it couldn't possibly be your ignition switch. I had a similar problem - the blower would come and go intermittently. I "exposed" it three different times - ugh. I was finally able to reproduce the problem on the test bench. If I tweaked the connectors, it would quit - probably a broken wire internally. $200 for a new blower motor with fans solved my problem.

The problem still looks like its in the blower case. I have to get a Volvo turbo wagon back on the road before I tear this one apart so it will have to wait for the weekend. The volvo needs a tailshaft bushing and seal replacement. Working on top of the benz will be a welcome relief after crawling under the volvo.

Life is grand when you have a driveway full of old cars, all calling out for your attention!!:p

Before you start tearing the car apart again, get a voltmeter and check the output voltage of your alternator. Some of the later model fan regulators have a thermal cutout if the control voltage gets too high and the regulator has to dissipate too much heat. I just went thru this with my 300E (BTW, thanks, Steve B.). My voltage regulator was acting up and finally failed. Check it first. Good Luck..

__________________
Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.